Document 1 through document 3 disclose self-diagnosis using capacitance of an electrochemical gas sensor. Document 1 and document 2 relate to a gas sensor using liquid electrolyte, and a pulse test signal is inputted into an amplifier rather than the gas sensor. The gas sensor is connected to another input of the amplifier in parallel with a fixed resistor, the amplifier changes its output according to the input of the test signal, and the transfer function of this process changes according to the capacity of the gas sensor. As the capacity of the gas sensor changes depending on whether the sensor is normal or not, the gas sensor can be diagnosed on the basis of the output of the amplifier.
In document 3, a pulse voltage is applied to a gas sensor, and self-diagnosis is made on the basis of the output waveform of the sensor after the end of the pulse. To be more specific, as the normal gas sensor has a large capacity and the defective gas sensor has a smaller capacity, the gas sensor can be checked on the basis of the relaxing speed of the output after the end of the pulse.
Document 1USP6251243Document 2USP6123818Document 3USP6200443